`env`: Consider adding `separator` option for `prefix` env vars

[lilyball]

Is your feature request related to a problem? Please describe.

I can define an env var with prefix to prefix a path onto a :-separated PATH-like variable. However there's no convenience for prefixing onto other types of variables. In my case I want to add something to NIX_CONFIG, which is a newline-separated variable. And today I have to write this like

{
  env = [{
    name = "NIX_CONFIG";
    eval = ''"builders = "${lib.escapeShellArg builderConfig}''${NIX_CONFIG:+$'\n'"$NIX_CONFIG"}'';
  }];
}

Describe the solution you'd like

I'd like to have a separator option which I can set to something other than ":".

This also requires some way to disable the realpath invocation. That makes sense for PATH-like prefixing but not for other variables. One possibility here is to say "if you define the separator option it disables path expansion, even if you set it to ":"". This would allow my definition above to look like

{
  env = [{
    name = "NIX_CONFIG";
    prefix = builderConfig;
    separator = "\n";
  }];
}

Describe alternatives you've considered

With this I'd also like to be able to suffix the variable too, not just prefix. That is possibly useful for PATH-like variables too, although much less common, but e.g. in my case I'd prefer to suffix instead of prefix to ensure my definition actually works even if the inherited $NIX_CONFIG already tries to define the key.

[zimbatm]

Why not? It makes me think of wrapProgram, maybe the env option should be modeled after its options, so there is some transferable knowledge there.